Many food products served in retail restaurants are fried in cooking oil or shortening either at atmospheric or elevated pressures. Many of such food products, such as chicken, seafood or vegetables are breaded before being placed in hot oil for frying. The breading is usually a granular wheat product loosely bound to the food by moisture and egg albumin so a variety of particles thereof of different sizes tend to become unattached during the frying process. At first these pieces float, but after substantial moisture is cooked out of them, they sink, falling to the bottom of the fry pot of the fryer where they burn to discolor and impart a burnt taste to the oil. The burnt particles have also been implicated in early breakdown of the cooking oil. The burning occurs even when the fry pot is designed to have a cool sump because the action of frying causes turbulence that keeps the oil at near the same temperature throughout the fry pot. For this reason many commercial food fryers are equipped with an oil filter, which is used to remove the breading particles from the oil after a certain number of frying cycles, the actual number which depending on the type and characteristics of the breaded food being fried. In the worst instances, the filtering of the cooking oil must be accomplished every three to six frying cycles.
Generally filtering is performed with the oil at or near normal frying temperatures, although some filter systems require that the oil be cooled to almost room temperature because of the filtering agents used. Typical oil filtering systems include a drain valve attached to the lower surface of the fry pot. A filter pan is placed underneath the drain valve and the valve is opened. The hot oil then pours into the filter pan with the operator taking care to avoid splashing that can result in severe burns. Even when the operator is careful, splashing can occur, so it is preferred that the operator take the time to dress in protective clothing prior to the commencement of the filtering process. Such protective clothing commonly includes oil and heat resistant gloves, an oil and heat resistant apron, oil and heat resistant boots, and safety glasses.
Once the oil starts to drain, various scrappers and brushes are used to clean the sides of the fry pot and the electric heating element or gas flame tubes. When all of the cooking oil is drained and the fry pot is partially clean, the drain valve is closed and the hot oil is pumped back into fry pot until it is about 1/3 full. This clean oil is then used to thoroughly wash the sides of the fry pot, after which the oil is again drained and passed through the filter to remove any particles that might have been dislodged during the cleaning operation. The clean oil is then pumped back into the fry pot, with the operator taking care that the pump does not go dry and pump air, since when air is pumped through hot oil, the air causes the oil to oxidize and degrade very quickly. After multiple filterings, the number of which are dependent upon the filter and the product accumulating therein, the filter must be changed. Generally this is done after the oil has been pumped back into the fry pot and the filter has had an opportunity to cool. The whole filtering operation even in an highly automated food fryer can take as much as half an hour, during which time the fryer is out of production. In filtering systems where the oil must be cooled prior to filtering, the fryer may be out of use for as much as four hours.
In retail operations where the fryer is not used to its full frying capacity, and only one filtering process per day need be performed. The filtering process can be performed as the last job of the day (for hot oil filtering) or the first job of the day (for cool oil filtering). However, if there is a level and heavy demand for fried food during the business day, a extra fryer must be purchased and operated just to cover the filtering time, or more than the recommended number of batches must be fried between filterings. In any case, maximum production is limited by the filtering operation.
From the above, it should be apparent that it is highly desirable that the number of filtering operations be held to an absolute minimum, since such can be dangerous and reduce the production of fryers by a substantial margin. For example, if a fry cycle takes 15 minutes including time to load and unload, then at least every hour and a half the machine will be off line for half an hour while the oil is being filtered, resulting in a quarter of the time during the day being unavailable for frying.
If the oil is not filtered regularly, then instead of being replaced after approximately 60 fry loads, its useful life maybe reduced to as low as 20 loads. For a retail fried food operation, it is important to extend the life of the oil as long as possible, not only because of the acquisition cost of the oil, but also because the disposal of used cooking oil is difficult and expensive. In addition, the oil comes in heavy plastic containers, which also must be disposed of. Theoretically, in the case of a chicken fry operation, the oil should last until so much fat has been rendered from the chicken that the cooking oil begins to froth and foam, at which time, the oil must be replaced.